One of the serious side effects of oral contraceptives is that of thromboembolism. We are evaluating the effects of oral contraceptives on the surface charge characteristics of the vascular system. When the surfaces of the blood vessels and blood cells become less negatively charged, conditions prone to thrombosis result. Electrokinetic techniques, such as cell electrophoresis, electroosmosis and streaming potential measurements (both in vitro and in vivo) are used to measure surface charge characteristics. The electrophoretic mobility of human platelets which are incubated with various contraceptives show little change from control values while platelets obtained from women taking an estrogen-progestogen pill showed a markedly lower electrophoretic mobility. Platelets obtained from women who were taking a progestogen- only contraceptives show little change in the electrophoretic mobility from control values. The current induced occlusion time of rat mesentery vessels is markedly lowered (compared to control values) by feeding rats an estrogen or an estrogen-progestogen combination; the feeding of progestogen only, also lowers the current induced occlusion time but not to the same degree. Work is underway to determine the effects of these contraceptives on blood vessel surface charge.